Renewable and clean energy in various forms, such as solar energy, wind energy, and electrochemical energy, is becoming increasingly important due to the pressure from both the environment and the human society. To this end, different types of energy storage and conversion devices, such as solar cells, fuel cells, thermoelectric generators, electrochemical supercapacitors, and rechargeable batteries, have been proposed and fabricated for facilitating energy utilization in a more sustainable and efficient way.
Compared with other types of renewable energy storage or conversion devices, electrochemical energy storage devices provide more reliable and stable energy output as well as ease of fabrication facilitating their large scale production. As a result, rechargeable batteries among all the electrochemical energy storage devices have been intensively investigated in the recent years. Accordingly, many different types of battery systems have been proposed, such as lithium-ion (Li-ion) batteries and sodium-ion (Na-ion) batteries which offer higher energy density as compared with supercapacitors. Among the various battery systems proposed, zinc-ion (Zn-ion) batteries, produced primarily from zinc (Zn) and manganese dioxide (MnO2), have received increased attention due to their safe nature resulting from the aqueous electrolyte and the stable Zn metal anode material utilized.
The demand for developing flexible and wearable electronics, however, is stimulating the desire for portable energy storage devices with high mechanical flexibility and high energy storage capabilities. Such flexible and high energy storage devices drive new requirements on the choice of materials for the development of suitable rechargeable batteries, such as for use in next-generation flexible and wearable electronics.
One approach to the fabrication of a flexible battery is to deposit electrochemical active materials onto a flexible substrate, such as carbon cloth, polymeric elastomers, or textiles, see X. Wang, F. Wang, L. Wang, M. Li, Y. Wang, B. Chen, Y. Zhu, L. Fu, L. Zha, and L. Zhang, Advanced materials, 2016, 28, 4904-4911, B. Liu, J. Zhang, X. Wang, G. Chen, D. Chen, C. Zhou, and G. Shen, nano letters, 2012, 12, 3005-3011, M. Koo, K. I. Park, S. H. Lee, M. Suh, D. Y. Jeon, J. W. Choi, K. Kang, and K. J. Lee, Nano letters, 2012, 12, 4810-4816, C. Wang, A. Ballantyne, S. Hall, C. Too, D. Officer, and G. Wallace, Journal of power sources, 2006, 156, 610-614, and W. Liu, Z. Chen, G. Zhou, Y. Sun, H. R. Lee, C. Liu, H. Yao, Z. Bao, and Y. Cui, Advanced materials, 2016, 28, 3578-3583, the disclosures of which are incorporated herein by reference. Implementation of such an approach generally renders the battery with mechanical flexibility under external forces, as compared with conventional batteries that comprise more rigid metallic current collectors, such as copper and stainless steel. However, although such batteries exhibit flexibility under external forces, they are relatively fragile and show negligible recoverability of performance once serious mechanical deformation of the battery has occurred.